Sensor Device and Method for Determining Properties of a Liquid

ABSTRACT

The invention relates to a sensor device and a method for detecting properties of a liquid. The liquid is accommodated in an inner chamber  14.  A capacitor arrangement  22, 26  in the inner chamber has spaced, opposing capacitor surfaces  24   a,    24   b,    28   a,    28   b  so that at least part of the liquid accommodated in the inner chamber 14 is arranged between the capacitor surfaces  24   a,    24   b,    28   a,    28   b.  An evaluation device  30  for supplying an output signal A depending on a capacitance value C 1,  C 2  of the capacitor arrangement  22, 24  comprises an excitation circuit  32  and an evaluation circuit  34.  The excitation circuit  32  has at least one measuring resistor R 1,  R 2,  R 1   a,  R 1   b  and means for applying an AC voltage to a series circuit consisting of the measurement resistor R 1,  R 2,  R 1   a,  R 1   b  and the capacitor arrangement  22, 24.  The evaluation circuit  30  has means for supplying the output signal A by measuring a voltage U 1,  U 2  across the capacitor arrangement  22, 24.

The invention relates to a sensor device and a method for determining properties of a liquid. In particular, the invention relates to a device and a method for detecting properties of a liquid using a capacitive measuring principle.

U.S. Pat. No. 6,269,693 B1 describes a capacitive sensor for measuring a property of a fluid or a fill level of a fluid in a container. A printed circuit board is either flexible or has rigid and flexible sections. Metal coatings form capacitor plates on the PCB which is then bent so that two capacitor plates are held at a distance from each other by spacers so that they form a capacitor. Additional metal coatings are provided for shielding. Tracks on the PCB connect the capacitor plates to an evaluation circuit, and the shielding to a reference potential. The bent PCB is held in a housing and fixed by fixing pins.

EP 1 106 997 A2 describes a method and a device for oil-in-water measurement. In an electric measuring cell, a capacitance is measured as a measure of an oil concentration in water flowing through.

The object can be considered that of proposing a sensor device and a method for detecting properties of a liquid in which accurate detection is possible with simple means, in particular for the determination of water content in oil.

The invention is solved by a sensor device according to claim 1 and a method for detecting properties of a liquid according to claim 13. Dependent claims refer to advantageous embodiments of the invention.

The sensor device according to the invention has an inner chamber for accommodating a liquid and a capacitor arrangement in the inner chamber. The inner chamber is preferably formed within a housing. It can for example be a container or tank; preferably, it is a piece of tube with an inlet and an outlet that are fluidically connected to each other across the inner chamber.

The capacitor arrangement comprises spaced, opposing capacitor surfaces with at least one gap so that at least part of the liquid accommodated in the inner chamber is arranged in the gap between the capacitor surfaces. The capacitor arrangement therefore preferably forms at least one measuring capacitance in which the liquid forms at least part of the dielectric.

According to the invention, an evaluation device is provided for supplying an output signal that is at least dependent on a capacitance value of the capacitor arrangement, preferably on the capacitance value of at least one measuring capacitance formed thereby. The measuring capacitance can be part of a complex impedance. The evaluation device comprises an excitation circuit and an evaluation circuit. It should be noted that in this case, the concept of the circuit should be understood functionally and not necessarily structurally, i.e., the excitation circuit and evaluation circuit can be realized in whole or in part with the same components and/or use common circuit parts. Likewise, the evaluation device, the excitation circuit, and/or the evaluation circuit can have additional elements and/or functionalities. Given a possible complete or partial digital design of a circuit or circuit parts, for example a common processor can thus realize both parts of the functionality of the excitation circuit as well as the evaluation circuit, as well as other functionalities, if applicable.

According to the invention, the excitation circuit has at least one measuring resistor and means for applying an AC voltage to a series circuit consisting of the measuring resistor and the capacitor arrangement, or respectively at least one measuring capacitance. thereof. The capacitor arrangement, or respectively preferably a measuring capacitance formed thereby, can for example be part of a complex impedance by mean of circuitry, connected for example together with at least one. measuring resistor as an RC element that is excited by an AC voltage with a suitable waveform and frequency. According to the invention, the evaluation circuit has means for supplying the output signal, at least by measuring the voltage across the capacitor arrangement. The voltage is therefore measured across the capacitor arrangement during or after excitation. The measurement, or respectively evaluation can for example include the phase of the voltage; however, the voltage level is preferably measured, in particular the peak value of the voltage.

In the method according to the invention, the liquid is accommodated in the inner chamber in which the capacitor arrangement is arranged, or it flows through it so that liquid is arranged between the capacitor surfaces. A voltage across the capacitor arrangement, or respectively a measuring capacitance is measured, and based on this, an output signal is generated that depends on properties of the liquid.

The device according to the invention and the method according to the invention therefore allow properties of the liquid accommodated in the inner chamber or flowing through it to be determined by evaluating electrical properties, for example according to a purely capacitive measuring principle, and/or by detecting properties of a complex impedance of which a measuring capacitance represents a part. Opposing capacitors surfaces of the capacitor arrangement form at least one measuring capacitance, wherein the liquid in the gap between the capacitor surfaces functions as a dialectic. Accordingly, the capacitance value of the measuring capacitance., and therefore of the capacitor arrangement, depend on the dielectric properties of the liquid that may deviate depending on the composition and type of the liquid. Accordingly, for example, a portion of water in oil that is arranged between the capacitor surfaces can be recognized.

By connecting the capacitor arrangement as an RC element and exciting with an AC voltage, highly accurate detection can be achieved with very simple means so that it is possible to even ascertain for example minute portions of water in oil.

According to an advantageous development of the invention, the series circuit forms a low-pass filter, and the frequency of the exciting AC voltage lies within the range of the cutoff frequency of the low-pass filter. In the simplest case of an RC element, the cutoff frequency is f_(C)=1/(2π RC), as is known. Preferably in this case, when determining the cutoff frequency, the capacitance value C is considered while the measuring capacitance is being filled with pure, liquid without impurities. Excitation “around” the cutoff frequency can for example be understood as meaning that the frequency of the AC voltage lies within the same magnitude as the cutoff frequency. Preferably, the measuring frequency, i.e., the frequency of the exciting AC voltage, lies somewhat higher than the cutoff frequency. As is known, with an ideal low-pass filter, the output level at the cutoff frequency is about 70% of the input level. A suitable range for the excitation frequency “around the cutoff frequency” could for example be considered a frequency range at which—only with reference to the RC low-pass filter and not to additional circuitry—the output level is between 10% and 90% of the input level, preferably 40-80%.

Preferably, the. evaluation circuit has a peak value detector so that a peak value of the AC voltage can be determined using the capacitor arranaement. The determined peak value can preferably be filtered by a low-pass filter and digitized in an analog/digital converter so that it can then be processed by a digital control unit. The processing can preferably be done by a program running on a processor. When using a sufficiently fast AID converter, peak value detection can, if applicable., also be. omitted, and the. particular momentary value of the AC voltage can be directly evaluated via the capacitor arrangement.

The excitation circuit is preferably designed so that the AC voltage (excitation voltage) is applied continuously with a constant waveform and/or frequency. Preferably, the excitation voltage is a square wave voltage with a constant voltage value. Accordingly, the measuring capacitance is cyclically charged and discharged, limited by the measuring resistance. In an alternative embodiment, a square wave signal with a constant current can be used as the excitation signal.

According to one development of the invention, the evaluation circuit can be designed to compare a peak value of the voltage across the capacitor arrangement with a threshold value, The comparison can be carried out in an analog circuit part, for example by means of a comparator, or by a digital circuit using a digitized value, in particular by a program that is executed on a computer processor. The threshold value can be fixed or determined. dynamically. In one potential embodiment, the threshold value is established by a statistical analysis of measurements in a defined state, in particular when the interior is filled with a pure liquid without impurities.

According to an advantageous development, two capacitor arrangements can each be provided as measuring capacitors in the inner chamber. A first capacitor arrangement with first capacitor surfaces is connected to the evaluation device which is designed to apply a first AC voltage to the series circuit consisting of a first measuring resistor and the first capacitor arrangement. A second capacitor arrangement with spaced, opposing second capacitor surfaces in the chamber is also connected to the evaluation device so that it can supply an output signal that also depends on the capacitance of the second capacitor arrangement. For this, the evaluation device can for example be designed with two channels, in particular can have a second excitation circuit that is designed to apply a second AC voltage to a series circuit consisting of a second measuring resistor and the second capacitor arrangement.

It is possible for the frequency of the first AC voltage, and the second AC voltage to be the same. Preferably, however, the frequencies can also differ, i.e. the measurement by the first capacitor arrangement therefore uses a different excitation frequency than the measurement by the second capacitor arrangement. This can on the one hand serve to enable a redundant sensor device, wherein operation at different frequencies reduces mutual influence on the one hand and, on the other hand, ensures that the two measurements are not influenced by interferences in the same way. As will be explained below with respect to advantageous embodiments, one of the two measurements can also be used to normalize, the other measurement. Particularly preferably, at least the first frequency can be chosen so that it lies above the cutoff frequency of the low-pass filter consisting of the first measuring resistor and the first capacitor arrangement.

In a preferred embodiment, the capacitor arrangement has a plurality of pairs of spaced opposing capacitor surfaces that each form measuring capacitances. The measuring capacitors are preferably electrically connected to each other such that they are electrically connected in parallel. The individual measuring capacitors can therefore be interconnected into a common, combined measuring capacitance. On the one hand, this increases the capacitance value which is good for the evaluation. On the other hand, a relatively large part of the inner chamber can therefore be captured simultaneously by ascertaining the capacitance value of the combined measuring capacitance.

In a preferred embodiment, the capacitor surfaces can be formed as conductor surfaces on circuit board sections. In so doing, congruent conductor surfaces can be provided on the front and rear side of one or all circuit board sections that are short-circuited by a direct electrical connection, i.e., kept at the same electrical potential. Accordingly, the material of the circuit board section does not function as a dielectric and does not influence the measurement.

In preferred embodiments, the capacitor arrangement can have a circuit board structure with several rigid circuit board sections which are each connected by flexible conductor track carrier sections. This is a flat, electrically nonconductive support material with electrically conductive conductor structures applied thereupon. In particular, suitable plastic materials can serve as the support material. The rigid circuit board sections preferably consist of epoxide resin with a glass fiber fabric, in particular FR4. The flexible conductor track carrier sections are ductile and can for example consist of polyimide. Conductor surfaces can be arranged on the rigid circuit board sections that form the capacitor surfaces. The conductor surfaces consist of conductive material such as metal, preferably copper. The flexible conductor track carrier sections preferably have tracks that are connected electrically to the capacitor surfaces.

When a circuit board structure with rigid circuit board sections and flexible conductor track carrier sections is used, there is a certain risk of a line break, in particular in the region of the flexible conductor track carrier sections. To be able to identify any damage to the flexible conductor track carrier sections, a circuit can be provided for recognizing a line break. This can in particular be connected to at least one conductor track that runs along an edge of a flexible conductor track carrier section since damage is naturally most likely to be feared in the edge region. Particularly preferably, the conductor tracks are correspondingly connected and therefore monitored on both sides of the flexible conductor track carrier sections running at the edge.

According to a development of the invention, a detection element can be arranged on one of the rigid ci re uit board sections. Such a detection element is preferably designed in such a way that it is recognizable by its electrical properties so that its electrical connection, or respectively the disconnection thereof, can be determined with a suitable detection circuit. Particularly preferably, the detection element is a detection resistor that, more preferably, can be electrically connected in parallel to the capacitor arrangement. The evaluation circuit can preferably have means for recognizing whether the detection resistor is or is not connected, Accordingly, damage can be recognized in that an electrical connection of the evaluation circuit to the detection resistor is interrupted.

Particularly preferably, the detection element can be connected to the evaluation circuit via a conduction path that extends over all flexible conductor track carrier sections. The integrity of all conductor track carrier sections is thereby checked.

In the following, embodiments of the invention will be further described with reference to the drawings. In the drawings:

FIG. 1 shows a schematic representation of a first, simplified embodiment of a sensor device;

FIG. 2 shows a schematic diagram of an evaluation circuit to which a sensor device is connected;

FIG. 3 shows a side view of a part of a PCB structure in an unfolded state,

FIG. 4 a, 4 b shows a longitudinal section and cross-section of a second embodiment of a sensor device with the circuit board structure from FIG. 3 in a folded form;

FIG. 5 shows a schematic representation of a circuit diagram of the electrical configuration of the circuit board structure in the second embodiment of the sensor device according to FIG. 4 a , 4 b;

FIG. 6 a, 6 b show circuit diagrams of a first and second variant for connecting the circuit board structure from FIG. 3-5 to an evaluation device;

FIG. 7 shows a diagram with a depiction of voltage signals depending on an excitation frequency related to an ideal low-pass filter,

FIG. 8 shows a diagram with a depiction of voltage signals depending on an excitation frequency related to the circuit from FIG. 5, 6 a, 6 b.

FIG. 1 shows a schematic representation of a sensor device 10 according to a first embodiment. A housing 12 with an inner chamber 14 is provided for accommodating a liquid that flows into an inlet 16, flows through the interior 14, and is discharged through an outlet 18.

The liquid can for example be oil in which unknown portions of water in the form of individual droplets can be contained as impurities. The sensor device to serves to detect any water content and output an output signal A that indicates the presence of water content.

This is accomplished according to a primarily capacitive measuring principle, wherein however a relevant measuring capacitance, as presented below, can also be part of a complex impedance, and other components of the complex impedance can be detected while measuring, or respectively detecting.

A capacitor rangement 20 is arranged in the inner chamber 14. The capacitor arrangement 20 comprises a first measuring capacitance 22 formed from two capacitor surfaces 24 a, 24 b facing each other across a gap 27, and a second measuring capacitance 26 formed from capacitor surfaces 28 a, 28 b that are arranged at the same distance from each other across the gap 27.

The liquid (oil) arranged in the gap 27 therefore forms the dielectric of the two measuring capacitances 22, 26. The capacitance value C1, C2 of the measuring capacitances 22, 26 depends on the properties of the liquid, in particular on any impurity, in this case for example by water droplets. The permittivity of oil is significantly lower than the permittivity of water so that the water droplets which flow through the gap 27 causes an increase in the capacitance values C1, C2 of the measuring capacitances 22, 26. Likewise, other components in the liquid that have dielectric properties deviating from the pure liquid can cause a detectable change.

The measuring capacitances 22, 26 in the direction of flow from the inlet 16 to the outlet 18 are arranged one after the other so that the fluid and the impurities transported therein can flow through them sequentially. Moreover, the measuring capacitances 22, 26 are arranged in the path of flow such that at least basically the entire liquid flow flows through the gap 27. This ensures that water components flowing through sequentially change the capacitance value of both measuring capacitors 22, 26.

The measuring capacitances 22, 26 are connected to an evaluation device 30 that, at given measuring intervals, determines a capacitance value C1 for the first measuring capacitance 22, and a capacitance value C2 for the second measuring capacitance 26 and, depending on the ascertained capacitance values C1, C2, emits the output signal A which indicates that a threshold value of the water content in the flowing oil has been exceeded.

FIG. 2 , in a schematic block diagram, shows functional elements of the evaluation device 30 that are connected to the measuring capacitances 22, 24. The evaluation device is designed with two channels, i.e., a separate channel is provided for each of the measuring capacitances 22, 24 and consists of an excitation circuit 32 that is connected to a series circuit consisting of a particular measuring resistor R1, R2 and the particular measuring capacitances 22, 24, and a channel of an evaluation circuit 34 that generates the output signal A.

Each measuring channel of the evaluation circuit 34 is connected to one of the measuring capacitances 22. 24 in order to determine their particular capacitance value C1, C2. Therefore, it has a buffer amplifier 36, a peak value detector 38, a low-pass filter 40 and an A/D converter 42 for each measuring channel. The two AM converters 42 are connected to a processor 44 on which a program is executed that processes signals U1, U2 from the A/D converter 42 and generates the output signal A therefrom.

The first and second measuring capacitances 22, 24 are each connected to the respective measuring resistor R1, R2 as an RC element so that a complex impedance is formed which is excited by the associated excitation circuit 32 with a square wave signal of a frequency f1, f2. The excitation frequencies f1, f2 of the two measuring channels differ from each other. As a result, the respective capacity C1, C2 is cyclically charged and discharged by the respective measuring resistor R1, R2. Given a fixed time sequence, the voltage U2 resulting via the respective measuring capacitances 22, 24 is dependent on the. capacitance value C1, C2.

The particular voltage signal U1, U2 is measured in the evaluation circuit 34, digitized, and processed. It is first buffered by the buffer amplifier 36. Its peak value is ascertained by the peak value detector 38 and evaluated — filtered by the low-pass filter 40—by the AID converter 42 as a digital signal that is fed to the processor 44.

The respective RC element at both measuring channels functions as a low-pass filter. FIG. 7 shows the curve of the output voltage Û1 depending on the frequency f as an example of the first measuring capacitor 22—idealized and double logarithmic. As shown therein, the curve of the peak value of the output voltage Û1 shown as a solid line results for a fixed capacitance value C1 (corresponding for example to the capacitance value of the first measuring, capacitance 22 filled with liquid without impurities) when excited by different frequencies f. Significantly below the cutoff frequency f_(G)=1/(2π R1C1), there is basically no damping and an essentially constant curve of the peak value of the output voltage Û1. Around the cutoff frequency f_(G), a voltage value Û1 results that decreases with the damping curve toward higher frequencies f.

However, the cutoff frequency f_(G) is dependent on the capacitance value C1 and therefore on the properties of the liquid, in this case, especially on the portion of potential water impurities in the flow of oil. Due to water content, the capacitance. C1 increases to an increased capacitance C1′, so that the cutoff frequency f_(G) decreases. in FIG. 7 , the dot-dashed line shows the curve of the peak value of the output voltage Û1 over various frequencies f for a slight water content in the gap 27. As shown therein, the curve is shifted to the left in comparison to conditions without impurities.

Accordingly, given a fixed frequency f1, there is a difference in the output signal Û1 from the change of the capacitance C1. to the capacitance C1′. To achieve adequate sensor sensitivity, the measuring frequency f1 is chosen so that a clear difference ΔU is indicated when there is a change. in capacitance. As is known, the output signal level at the cutoff frequency fg is about 70%. The measuring frequency ft can for example be chosen so that the output signal level is between 90% and 10% (in each case given a fixed capacitance C1 without contamination).

When the measuring frequency, or respectively excitation frequency, f1 of the first channel of the evaluation device 30 is appropriately chosen, a variable output signal Û1 then results depending on whether there is pure liquid (oil) without water content or a certain portion of water droplets in the gap 27. Likewise., other kinds of impurities such as for example metal chips cause a change in the complex impedance, which is discernible from a changed output signal.

The two separate measuring capacitances 22, 24 and each of the assigned channels of the evaluation circuit 30 therefore. supply signals Û1, Û2 in digital form to the processor 40. The two signals Û1, Û2 are processed by an evaluation program executed by the processor 44 to generate the output signal A.

This includes on the one hand a decision as to whether the respective signal Û1, Û2 manifests a deviation that indicates water content in the flowing oil. On the other hand, the processing can also include a plausibilization of the respective signal to identify potential error states.

In a preferred embodiment, the evaluation program executed by the processor 44 performs a threshold value comparison for at least one of the signals Û1, Û2, wherein when a previously set threshold is undershot, a change is detected such that a contamination of the oil by water is indicated in the output signal A. The respective threshold can on the one hand be specified according to previous calculations or, on the other hand, ascertained by measurements and possibly statistical evaluations.

Accordingly, for example in a new state in which one can assume that only pure oil is in the gap 27, a frequency distribution of a plurality of measurements can be recorded, and thus for example the statistical parameters of a standard distribution (average, standard deviation, sigma) can be ascertained. Depending thereupon, the. decision threshold can then be established for example as an average minus n*Sigma, wherein the factor n is to be appropriately selected so that, on the one hand, sufficient sensitivity is achieved and, on the other hand, sufficient robustness against false detections remains guaranteed.

When the sensor device to is operating, the two channels of the evaluation device 30 can be operated with the same or different frequencies f1, f2, In an embodiment with the same frequencies f1, f2, crosstalk can arise between the channels in certain circumstances; frequencies that at least slightly differ from each other may therefore be preferable. Both frequencies ft, f2 can be in the range of the cutoff frequency of the particular RC element (wherein the capacitance C without impurities is significant). The signals Û1, Û2 from the two channels can each be independently used for a separate measurement and detection of significant impurities. The. detection of the two channels can then be plausibilized so that, for example, the presence of an impurity is only signaled if impurities are simultaneously detected in both channels.

In alternative embodiments, the frequencies f1, f2 can also deviate more strongly from each other. For example., the first channel connected to the first measuring capacitance 22 can be operated at a frequency f1 in the range of the cutoff frequency of the particular R1C1 element (wherein the capacitance C1 without impurities is also significant in this case). The second channel connected to the second measuring capacitance 24 is then for example operated at an excitation frequency f2 significantly below the excitation frequency f1 and also below the cutoff frequency, i.e., in ranges in which the curves shown in FIG. 7 have no or little changes with the frequency. In in this case, the signal Û2 created in the second channel can be used to normalize the signal at, for example taking the difference or forming the ratio of the two signals.

In addition, it is noted that the capacitance values C1, C2 are temperature-dependent. A temperature sensor (not shown) is therefore preferably arranged in the inner chamber 14 of the housing 12 whose measuring signal is also fed to the processor H. The evaluation program then takes into account a compensation curve that has been previously calculated or experimentally ascertained depending on the temperature signal.

Whereas the above-described first embodiment of the sensor device only forms the basic form, various other embodiments are possible. In the following, a second embodiment of a sensor device will be described with reference to FIG. 3 , FIG. 4 a and FIG. 4 b . The second embodiment provides a special arrangement of capacitor surfaces of the measuring capacitances 22, 26 in the inner chamber 14 of the housing 12 as will be explained below in detail. In all other respects, however, the sensor device according to the second embodiment corresponds to the above-describe sensor device to according to the first embodiment, in particular with respect to the evaluation device 30. In the following, the differences between the embodiments will therefore be explained, wherein identical reference signs will be used for the same or directly comparable elements.

FIG. 4 a, 4 b show a cross-section and longitudinal section of a part of a sensor device no according to the. second embodiment. This includes a two-part housing 12 with a housing top part 12 a and a housing bottom part 12 b, between which the inner chamber 14 is formed. A. capacitor arrangement 120 is arranged in the inner chamber 14. Each of the housing halves 12 a, 12 b has a holding structure 60 with holding elements 62 projecting in the direction of the inner chamber 14 and slots 64 formed therebetween.

FIG. 3 shows a part of the condenser arrangement 120. It includes a circuit board structure with several rigid circuit board sections 52 a, 52 b—five in the shown example—that are each connected by flexible conductor track carrier sections 54 into a chain. The rigid circuit board sections 52 a, 52 b are for example routine FR4 printed circuit boards that are. flexible conductor track carrier sections 54, such as polyimide strips.

The flexible conductor track carrier sections 54 are each attached to the edges 58 of the rigid circuit board sections 52 a, 52 b and are fixed thereto. The flexible conductor track carrier sections 54 in the shown embodiment are arranged not in the middle but at the border of the respective edge 58. The flexible conductor track carrier sections 54 are lesser in width in comparison to the length of the edges 58 so that most of the length of the edges 58 remains free, and the corresponding regions of the rigid circuit board sections 52 a, 52 b can be used for fastening, as will be explained further below.

Large conductor surfaces 24 a, 24 b, 28 a, 28 b are arranged on each of the rigid circuit board sections 52 a, 52 b on both sides. Two conductor surfaces 24 a, 28 a, or respectively 24 b, 28 b are arranged next to each other on the front and back side of each of the rigid circuit board sections 52 a, 52 b. The conductor surfaces 24 a, 24 b, 28 a, 28 b are formed like the conductor tracks 56 on the flexible conductor track carrier sections 54 and the conductor tracks on the rigid circuit board sections 52 a, 52 b and as copper layers. The conductor tracks connect the conductor surfaces 24 a, 24 b, 28 a, 28 b electrically as schematically shown in FIG. 5 .

To be arranged in the inner chamber 14, the circuit board structure of the capacitor arrangement 120 is folded so that the rigid PCB sections 52 a, 52 b are each arranged in parallel at a distance from each other, and the conductor surfaces 24 a, 24 b, 28 a, 28 b function as capacitor surfaces and form measuri ng capacitances across the gaps arranged therebetween.

The circuit board structure of the capacitor arrangement 120 is held in the folded arrangement within the housing 12 in that the edges 58 of the rigid circuit board sections 52 a, 52 b are each inserted into the slots 64 of the holding structure 60 where they are accommodated in a tight fit and are fixed both by the holding elements 62 as well as the holding pins penetrating the rigid circuit board sections 52 a, 52 b. As shown in FIG. 4 b, however, only the free sections of the edges 58, i.e., not occupied by the flexible conductor track carrier sections 54, are accommodated in the slots 64 so that the flexible conductor track carrier sections 54 are not clamped.

As schematically shown in FIG. 5 , the conductor surfaces 24 a, 24b; 28 a, 28 b applied on both sides of the rigid circuit board sections 52 a, 52 b are electrically short-circuited, i.e., the conductor surfaces on the front and back side are always at the same electrical potential. As already explained, opposing conductor surfaces 24 a, 24 b; 28 a, 28 b form measuring capacitancesacross the gaps arranged therebetween, wherein the liquid accommodated in the inner chamber14 forms the dielectic. The direct electrical connection of the conductor surfaces 24 a, 24 b; 28 a, 28 b on both sides ensures that the material of the rigid circuit board sections 52 a, 52 b does not form a dielectric of the measuring capacitances and therefore does not have any influence on the measurement.

As moreover shown in FIG. 5 , the individual measuring capacitances are connected in parallel so that two combined measuring capacitances are formed that each of which covers all of the gaps and therefore the entire inner chamber 14. As shown in FIG. 1 for the basic embodiment, the two combined measuring capacitors are arranged sequentially between the inlet and outlet (not shown).

FIG. 6 a shows a first version of the connection of the capacitor arrangement 120 to the evaluation device 30. As already explained with respect to FIG. 2 , the combined measuring capacitor C1 is connected in series by a resistance element to the excitation circuit 32.

A complex impedance can, for example, be formed in various ways in that the resistance element R1 a is arranged in front of the measuring capacitance from the perspective of the. excitation circuit 32, or alternatively behind it (resistance element R1 b), or both shown resistance elements R1 a, R1 b can be provided.

In addition, to detect line interruptions, in particular in the region of the flexible conductor track carrier sections 54, a conductor loop can be formed over all rigid circuit board sections 52 a, 52 b and over all flexible conductor track carrier sections 54 by connecting in parallel a detection resistor Rd to the measuring capacitance C1. The detection resistor Rd can be arranged on the last rigid circuit board section sea, 52 b as shown in the first version according to FIG. 6 a , or in another location, for example on the first rigid circuit board section 52 a as shown in the second version according to FIG. 6 b . In any case, a conductor loop containing the detection resistor Rd is formed over all the flexible conductor track carrier sections 54, wherein preferably, the conductor tracks of the conductor loop each run along the two edges of the flexible conductor track carrier sections 54 so that they are monitored in a special way.

A complex impedance is formed by connecting in parallel the detection resistor Rd to the combined measuring capacitance in combination with connecting in series to the resistance elements R1 a andjor R1 b. The evaluation is nonetheless carried out as explained above, wherein however when the conductor loop is no longer connected to the detection resistor Rd because of a line break, this is recognized by the evaluation device 30.

For the complex impedance that is formed by R1 a/R1 b, the capacitance C1 and the detection resistor Rd, FIG. 8 shows an example of the curve of the output voltage. U1 depending on the frequency 1. For a capacitance value C1 (without impurities), the frequency response with a cutoff frequency for example at f_(G)=1/(2πR1C1) shown with a solid line results, whereas the frequency response shown with a dot-dashed line results when there is water content in the gap 27 with a lower cutoff frequency, i.e., as a curve shifted to the left in comparison to conditions without impurities.

The respective, frequency--dependent difference of the output signals between the relevant instance of liquid without impurities (solid line) and the liquid with water droplets (dot-dashed line) is shown as a dotted line. The difference curve forms a maximum in the region slightly above the cutoff frequency f_(G), for the instance without impurities.

This frequency at which the voltage difference is at a maximum which arises depending on the change in capacitance is used as the preferred excitation frequency f₁ of the first channel of the. evaluation device 30. As plotted for example in FIG. 8 , an output voltage Ua arises at this measuring frequency f, when there is liquid without water content, whereas the output voltage Ub results when there is a certain portion of water droplets.

Whereas the frequency response is variable in the range of the cutoff frequency as shown, the signal curves significantly above, or respectively below the cutoff frequency are very flat. The voltage values U1 at low frequencies (about 5 V in the example of FIG. 8 ) and at high frequencies (about 2.7 V in the example of FIG. 8 ), are inter alia determined by the detection resistor Rd. Voltage values significantly outside of the voltage range limited in this way (2.7-5 V in the example) accordingly indicate an error state, i.e., such as a short-circuit or line break. This is easily discernible by an evaluation program executed by a processor 44.

It is noted that the invention is not restricted to the described embodiments and versions; instead, other embodiments are possible. Accordingly, for example, only a single measuring capacitance or several measuring capacitances can be provided instead of two measuring capacitances 22, 26. A different number of rigid circuit board sections 52 a, 52 b can also be provided, for example. Instead of signal evaluation with peak value detection, the particular momentary value can also be evaluated by using faster A/D converters, in general, the features of the embodiments and the claims can be combined as desired. 

1. A sensor device having: An inner chamber (14) for accommodating a liquid, a capacitor arrangement (22, 26) in the inner chamber (14), wherein the capacitor arrangement (22, 26) has spaced, opposing capacitor surfaces (24 a, 24 b, 28 a, to 28 b) so that at least part of the liquid accommodated in the inner chamber (14) is arranged between the capacitor surfaces (24 a, 24 b, 28 a, 28 b), an evaluation device (30) for supplying an output signal (A) that is at least dependent on a capacitance value (C1, C2) of the capacitor arrangement (22, 24), wherein the evaluation device (30) comprises an excitation circuit (32) and an evaluation circuit (34), wherein the excitation circuit (32) has at least one measuring resistor (R1, R2, R1 a, R1 b) and means for applying an AC voltage to a series circuit consisting of the measuring resistor (R1, R2, R1 a, R1 b) and the capacitor arrangement (22, 24), and wherein the evaluation circuit (30) has means for supplying the output signal (A) by measuring a voltage. (U1, U2) across the capacitor arrangement (22, 24).
 2. The sensor device according to claim 1, wherein: the series circuit consisting of the measuring resistor (R1, R2, R1 a, R1 b) and the capacitor arrangement (22, 24) form a low-pass filter, and the AC voltage is applied at a frequency (f1, f2) around the cutoff frequency of the low-pass filter.
 3. The sensor device according to one of the preceding claims, wherein: the evaluation circuit (30) has a peak value detector (38).
 4. The sensor device according to one of the preceding claims, wherein: the evaluation circuit (30) is designed to compare a peak value (Û1, Û2) of the voltage across the capacitor arrangement (22, 24) with a threshold value,
 5. The sensor device according to one of the preceding claims, wherein: the capacitor arrangement is a first capacitor arrangement (22) with first capacitor surfaces (24 a, 24 b), and the evaluation device is a first evaluation device. with a first excitation circuit (32) that applies a first AC voltage to the series circuit consisting of a first measuring resistor (R1) and the first capacitor arrangement (22), and in the inner chamber (14), a second capacitor arrangement (24) is arranged with spaced, opposing second capacitor surfaces (28 a, 28 b), wherein the evaluation device for supplying an output signal dependent on a capacitance value (C2) of the second capacitor arrangement (24) as well has a second excitation circuit (32) that applies a second AC voltage to a series circuit consisting of a second measuring resistor (R2) and the second capacitor arrangement (24), wherein the first AC voltage has a first frequency (f1), and the second AC voltage has a second frequency (f2), wherein the first frequency (f1) differs from the second frequency (f2).
 6. The sensor device according to one of the preceding claims, wherein: the capacitor arrangement (22, 24) has a plurality of pairs of spaced, opposing capacitor surfaces (24 a, 24 b, 28 a, 28 b) that each form measuring capacitances, wherein the measuring capacitances are connected to each other such that they are electrically connected in parallel.
 7. The sensor device according to one of the preceding claims, wherein: at least one of the capacitor surfaces is formed as a conductor surface on a circuit board section.
 8. The sensor device according to claim 7, wherein: a first conductor surface is arranged on a front side of the circuit board section, and a second conductor surface is arranged on an opposite back side, wherein the first and second connector surface (24 a, 24 b) are at the same electrical potential by direct electrical connection.
 9. The sensor device, according to one of the preceding claims, wherein: the capacitor arrangement has a circuit board structure (120) with several rigid circuit board sections (52 a-g) which are each connected by flexible conductor track carrier sections (56), wherein conductor surfaces (24 a, 24 b, 28 a, 28 b) that form the capacitor surfaces are arranged on the rigid circuit board sections, and wherein the flexible conductor track carrier sections (54) have conductor to tracks (56) that are connected electrically to the capacitor surfaces (24 a, 24 b, 28 a, 28 b).
 10. The sensor device according to claim 9, wherein: a detection element (Rd) is arranged on at least one of the rigid PCB sections, and the evaluation circuit (30) has means for detection whether or not the detection element (Rd) is connected to the evaluation circuit.
 11. The sensor device according to claim 10, wherein: the detection element is a detection resistor (Rd) that is electrically connected in parallel to the capacitor arrangement (22, 24).
 12. The sensor device according to claim 10 or 11, wherein: the detection element (Rd) is connected to the evaluation circuit (30) via a conduction path that extends over all flexible conductor track carrier sections (54),
 13. A method for determining properties of a liquid, wherein: the liquid is accommodated in an inner chamber (14), wherein at least one capacitor arrangement (22, 24) is arranged in the inner chamber (14), wherein the capacitor arrangement (22, 24) has spaced, opposing capacitor surfaces (24 a, 24 b, 28 a, 28 b) so that at least part of the liquid accommodated in the inner chamber (14) is arranged between the capacitor surfaces (24 a, 24 b, 28 a, 28 b), wherein an AC voltage is applied to a series circuit consisting of a measuring resistor (R1, R2, R1 a, R1 b) and the capacitor arrangement (22, 24), and wherein a voltage (U1, U2) across the capacitor arrangement (22, 24) is measured, and based on this, an output signal (A) is generated that depends on properties of the liquid. 